1. Field of the Invention
The present invention relates to a photoconductive type sensor and its driving and, more particularly, to a photoconductive type sensor and its driving method and apparatus in which the signal current is large and the response speed to light is improved.
A photoconductive type sensor and its driving method and apparatus according to the invention are applied to, for example, the image input section and the like of a facsimile, digital copier, image reader, or the like.
2. Related Background Art
Recently, in association with the spread of what are called electronic office machines such as facsimile machines, digital copiers, and the like, the demand for a small-sized and low-cost image input apparatus grows more and more. For this purpose, attention has been directed to the line sensor of the equal magnification type which can be directly brought into contact with an original and in which the image forming system is unnecessary or the operation distance of the image forming system is short.
Since the equal magnification type line sensor needs to have a length which is equal to the length of one side of the actual original, it is necessary to densely arrange a number of sensors in order to obtain high resolution. To attain this object, it is suitable to use a thin film semiconductor such as CdS.Se, or amorphous silicon hydrided (hereinafter, referred to as an a-si:H) or the like.
The photosensors using such a thin film semiconductor are mainly classified into two kinds the photodiode type and the photoconductive type.
In the case of the photodiode type, since a reverse bias voltage is applied between, the electrodes, the electron-hole pairs generated by the light respectively reach both electrodes and a current merely flows in the opposite direction, and thereafter no carriers are implanted from the electrodes.
On the other hand, in the case of the photoconductive type, since electrons or holes can be implanted from the electrodes, the density of electrons or holes in the semiconductor sufficiently increases. By applying a voltage between the electrodes, a fairly large output current can be obtained as compared with the photodiode type.
FIG. 1 is a schematic constitutional diagram of a conventional photoconductive type sensor.
In the diagram, a semiconductor layer 2 such as CdS.Se, a-Si:H or the like is formed as a photoconductive layer on an insulative substrate 1 made of glass, ceramics, or the like. Further, a pair of electrodes 4 and 4' are formed over the semiconductor layer 2 through doping semiconductor layers 3 and 3' for ohmic-contact. If the carriers which are implanted from the electrodes into the semicondutor layer 2 are electrons, the doping semiconductor layers 3 and 3' are of the n type, and if the carriers are holes, the doping semiconductor layers 3 and 3' are of the P type.
In such a constitution, when light enters from the side of the substrate 1 (in the case of the transparent substrate 1) or from the sides of the electrodes 4 and 4', the density of electrons or holes which will contribute to conduction rises in the semiconductor layer 2 between the electrodes 4 and 4' by the light excitation. Therefore, as shown in the diagram, by applying a voltage between the electrodes 4 and 4', a large signal current flows by the light incidence. Thus, a large output can be obtained from both ends of a load resistance (not shown).
Since a large output signal is derived as mentioned above, a load of a signal reading circuit of the photoconductive type sensor is lightened and the equal magnification type line sensor or the like using the photoconductive type sensor can be constituted as a sensor having high sensitivity and a low cost.
However, since the response speed of the foregoing conventional photoconductive type sensor to the light is insufficient, there is a problem such that, for example, in the case of constituting the equal magnification type line sensor having a number of pixel sensors, the original cannot be read at a high speed.